1. Field of the Invention
The present invention relates to a multi-axis magnetic lens and variants thereof which can focus a plurality of charged particle beams individually and in parallel. More particularly, it relates to an apparatus of plural charged particle beams with two or more multi-axis magnetic lenses, which can inspect and/or review defects on a wafer or a mask with high resolution and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on a wafer and or a mask during fabrication processes, which reduce the yield to a great degree. As requirements on chip performance are rising, smaller and smaller critical feature dimensions of patterns have to be used, and hence the yield management tools with optical beam have gradually become incompetent. Meanwhile, the yield management tools, each of which is based on the principle of scanning electron microscope (SEM) with a single electron beam, have been more and more used to inspect or review the defects or particles. The reason is that an electron beam can offer superior spatial resolution compared to a photon beam due to its short wavelength. However, such a superior spatial resolution will be fundamentally deteriorated by electron interaction or called as Coulomb Effect as the single electron beam current is increased to obtain a high throughput competent for mass production.
For mitigating the limitation on throughput, a promising solution is to use a plurality of electron beams each with a small current instead of using a single electron beam with a large current. For this solution, the plurality of electron beams can be respectively focused by a plurality of single-axis magnetic/electrostatic lenses in a conventional manner or a plurality of magnetic sub-lenses of a multi-axis magnetic lens as Maekawa et al first proposed in the U.S. Pat. No. 3,715,580 as early as in 1971. Compared with the first way, the later way can even halve the interval between every two adjacent beams, thereby almost doubling the throughput.
FIGS. 1A and 1B respectively illustrate the configuration and the magnetic field distribution of the conventional multi-axis magnetic lens 100 proposed in U.S. Pat. No. 3,715,580. When an electric current is exerted into the common excitation coil 44, an axisymmetric magnetic field (round-lens field) will be formed by one of a plurality of pairs of coaxial-through-round holes inside the parallel magnetic conductor plates 41 and 42 and distribute along the coincident central axes thereof (such as 31 in FIG. 1B), and consequently one magnetic sub-lens such as 30 is formed thereby. The yoke 43 surrounds the common excitation coil 44 to reduce the magnetic resistance so that more magnetic flux will leak out through each pair of coaxial-through-round holes.
There are two issues deteriorating the performance of the conventional multi-axis magnetic lens. The magnetic flux leaked out through each pair of coaxial-through-round holes depends on the position thereof, geometrical shapes and magnetic permeability of the plates 41 and 42, and the distribution of all the pairs of coaxial-through-round holes. Hence, as the first issue, each magnetic sub-lens further comprises a lot of non-axisymmetric transverse magnetic field components or called as high order harmonics (such as dipole field and quadrupole field). Only the round-lens field is necessary for focusing an electron beam, and the other components are undesired due to generating additional aberrations. To compensate the influence of each high order harmonic, at least one additional element generating the same type field component is required and thereby increasing the volume and the complication of the multi-axis magnetic lens obviously. As the second issue, all the magnetic sub-lenses are different in the round-lens fields even if all the pairs of coaxial-through-round holes are same in geometry. A magnetic sub-lens closer to the geometrical center of the plates 41 and 42, has a weaker round-lens field. For example, the magnetic sub-lens 20 has a weaker round-lens field than the magnetic sub-lens 10. The differences in round-lens field incur the differences in beam focusing with respect to a specific imaging plane. That means the beams 1, 2 and 3 respectively passing through the magnetic sub-lenses 10, 20 and 30 will not be focused onto a same plane even if they are incident in the same situation.
Chen et al propose one method in U.S. Pat. No. 8,003,953 and the first application of the cross reference to fundamentally mitigate or even eliminate the high order harmonics per se of each magnetic sub-lens and the differences among round-lens fields of all the magnetic sub-lenses. The method comprises three principal steps as expressed by the multi-axis magnetic lens 200 shown in FIG. 2A. The first step, as the most profound step, is inserting a magnetic round ring (such as 12) inside each through hole of every magnetic sub-lens (such as 10) with a radial gap (such as 14). The radial gap can be vacuum or filled of non-magnetic or weakly-magnetic material, which keeps a strong magnetic coupling between the magnetic round ring and the corresponding magnetic conductor plate and effectively weakens the non-axisymmetry of the distribution of magnetic scalar potential inside the magnetic round ring. Consequently, a magnetic field will be leaked out through the magnetic-circuit gap (such as 15) between the two magnetic round rings (such as 12 and 13) and distributes along the coincident central axes (such as 11) thereof. Out of the magnetic field, the axisymmetric component (round-lens field) is strong enough and the non-axisymmetric transverse field components are almost eliminated. With this way, the through holes are not necessary round in shape. The magnetic sub-lens module with respect to the magnetic sub-lens therefore is formed by the pair of through holes, the magnetic round rings therein, the radial gaps therebetween and the magnetic-circuit gap. The magnetic round rings functionally are magnetic pole-pieces and can flexibly shape the magnetic circuit gap for a specific application, such as an axial magnetic-circuit gap of a magnetic condenser sub-lens or a radial magnetic-circuit gap of a magnetic immersion objective sub-lens.
The second step is extending one of the two magnetic round rings of each magnetic sub-lens into the other so as to further eliminate the high order harmonics therebetween. If the extended magnetic round ring fully goes through the through hole in which the other magnetic round ring is inserted, in some cases the other magnetic round ring can even be removed for the sake of simplification in manufacturing. In the sub-lens 10 of the multi-axis magnetic lens 201 shown in FIG. 2B, the upper magnetic round ring 12 fully crosses over the upper and lower through holes. When working as an immersion objective lens with very short working distance (the gap between the objective lens and the sample), the lower magnetic round ring 13 therefore can be removed for the sake of simplification in manufacturing. The third step, as one global magnetic-shielding means, is placing two magnetic-shielding plates 51 and 52 respectively above and below the two parallel magnetic conductor plates 41 and 42 shown in FIG. 2A so as to reduce the high order harmonics of every magnetic sub-lens in the areas above and below the multi-axis magnetic lens 201 respectively. Furthermore, the differences of the radial gaps of all the magnetic sub-lens modules can be specifically designed to mitigate or even eliminate the round-lens field differences among all the magnetic sub-lenses.
Therefore, each magnetic sub-lens of the multi-axis magnetic lenses proposed by Chen et al can provide a performance as good as a conventional single-axis magnetic lens. In addition, Chen et al further propose a way to keep the performance stable for a specific application, as shown in FIG. 3 and disclosed in the third application of the cross reference. Instead of using the common excitation coil to provide a common magnetic flux, an annular permanent-magnet unit 45 provides a basic magnetic flux to all the magnetic sub-lenses modules and a plurality of subsidiary coils (such as 18) is respectively excited to provide an additional magnetic flux to one of the magnetic sub-lens modules (such as 10). This configuration weakens the factors which vary the magnetic field of each magnetic sub-lens, such as the variation of the coil excitation (product of coil turns T and coil current I) and thermal deformation. For the sake of clarity, a modified multi-axis magnetic lens means any of the multi-axis magnetic lenses proposed by Chen et al. and a conventional multi-axis magnetic lens means any of the other prior-art multi-axis magnetic lenses hereafter.
Generally speaking, a multi-axis magnetic lens can be used as an objective lens, a condenser lens and a transfer lens and hence an apparatus of plural charged particle beams can use one or more multi-axis magnetic lenses such as U.S. Pat. Nos. 7,262,418, 7,253,417 and 6,787,780. However, for a conventional multi-axis magnetic lens, the impact of the first issue mentioned above will be more sever when functioning as a condenser lens or a transfer lens than as an objective lens due to incurring much larger off-axis aberrations on each beam path. Comparably, a modified multi-axis magnetic lens can perform better because it can effectively mitigate or even eliminate the first issue. Accordingly, an apparatus of plural charged particle beams, which employs two modified multi-axis magnetic lenses as the objective lens and the condenser lens respectively, will provide higher resolutions and higher throughputs than those of the yield management tools of the prior art. For the sake of simplification and clarity, hereafter a multi-axis magnetic objective lens means a multi-axis magnetic lens whose magnetic sub-lenses respectively functions as an objective lens, while a multi-axis magnetic condenser lens means a multi-axis magnetic lens whose magnetic sub-lenses respectively functions as a condenser objective lens.